(a) Field of the Invention
The present invention relates to an improved separator vessel for separating and removing sulphate soap from black liquor obtained from a sulphate pulping process
(b) Description of Prior Art
A brief description of the state of the art relative to soap removal from black liquor in sulphate pulping process follows. Organic acids, such as resin acids and fatty acids are present in wood chips. Chips made out of pine wood, are found to contain more of the mentioned acids than other wood species. The alkaline conditions in the sulphate (kraft) pulping process convert these resin and fatty acids to their sodium salts which "salt out" from the black liquor (cooking liquor) separated from the pulp after cooking process. This material is called soap skimming which are separated from the top of the black liquor in a special vessel called skimming vessel. However, the soap separation in the skimming vessels is not complete. The soap still includes some black liquor.
The skimmings (with some included black liquor) are commonly known as the raw sulphate soap. The raw sulphate soap is, after separation of its remaining black liquor content, dissolved in sulphuric acid to form tall oil. The tall oil consists of approximately 40-50% of each fatty and resin acids with about 10% other acids. Tall oil yields are about 40-200 lb/ton of pulp. This material is not of much commercial utility until it is separated in two major component classes, fatty and resin acids by vacuum distillation. Fatty and resin acid fractions of 90-98% purity are obtainable. Refined tall oil is used to make detergents, resin glue, etc. for use of the industry.
The soap is usually separated from two places in the black liquor cycle of the sulphate pulping process. Firstly, from weak black liquor after liquor separation from pulp and before the liquor evaporation, and secondly, from strong black liquor between evaporator units and/or after completing the evaporation.
By increasing the black liquor concentration, the soap removal can be improved. The weak black liquor concentration is usually 11.0-17.0% of solids and the strong black liquor concentration is usually 50-65% of solids after evaporation. In systems where soap is skimmed between evaporator units at liquor concentration of 25-35%, an improved evaporator operation can be achieved. At this concentration, the soap removal is very effective and can be further improved by proper temperature and density control.
In the kraft pulping industry, several concepts are in use to design soap skimming vessels. However, these vessels have numerous problems such as that they require high initial capital cost, yield low quality soap, have low soap removal efficiency, and require high maintenance cost.